Problem: Before the previous stop there were 12 people riding on a train. 4 people ended up getting off at the stop. How many people are riding the train now?
Solution: The number riding the train now is the difference between the number who were riding and the number who got off at the previous stop. The difference is $12 - 4$ people on the train. $12 - 4 = 8$ people are on the train.